Antitachycardial pacing

ABSTRACT

Protocols for antitachycardial pacing including biphasic stimulation administered at, or just above, the diastolic depolarization threshold potential; biphasic or conventional stimulation initiated at, or just above, the diastolic depolarization threshold potential, reduced, upon capture, to below threshold; and biphasic or conventional stimulation administered at a level set just below the diastolic depolarization threshold potential. These protocols result in reliable cardiac capture with a lower stimulation level, thereby causing less damage to the heart, extending battery life, causing less pain to the patient and having greater therapeutic effectiveness. In those protocols using biphasic cardiac pacing, a first and second stimulation phase is administered. The first stimulation phase has a predefined polarity, amplitude and duration. The second stimulation phase also has a predefined polarity, amplitude and duration. The two phases are applied sequentially. Contrary to current thought, anodal stimulation is first applied and followed by cathodal stimulation. In this fashion, pulse conduction through the cardiac muscle is improved together with the increase in contractility.

RELATED APPLICATION DATA

The present disclosure is a continuation-in-part application related tothe U.S. patent application entitled “Augmentation of ElectricalConduction and Contractility by Biphasic Cardiac Pacing”, Ser. No.08/699,552, filed Aug. 19, 1996, now U.S. Pat. No. 5,871,506.

FIELD OF THE INVENTION

The present invention relates generally to implantablecardioverter/defibrillator with antitachycardial pacing capabilitiesand/or a method of such pacing.

BACKGROUND OF THE INVENTION

The typical implantable cardioverter/defibrillator (ICD) delivers aninitial electrical countershock within ten to twenty seconds ofarrhythmia onset, thereby saving countless lives. Improved devices haveantitachycardia pacing capabilities in addition tocardioverting/defibrillating functions. These ICDs are capable ofdifferent initial responses to one or more tachycardia as well as aprogrammable sequence of responses to a particular arrhythmia.

The output energy level is generally set by a physician in accordancewith a patient's capture threshold, determined at the time of heartimplantation. This threshold represents the minimum pacing energyrequired to reliably stimulate a patient's heart. However, due to traumaassociated with the stimulation, scar tissue grows at the interfacebetween the implanted cardiac pacer leads and the myocardium. This scartissue boosts the patient's capture threshold. To insure reliablecardiac capture, the output energy level is thus generally set at alevel which is a minimum of two times greater than the initiallymeasured capture threshold. A drawback to such an approach is that thehigher stimulation level causes more trauma to the cardiac tissue thanwould a lower level of stimulation, and hence promotes the formation ofscar tissue, thereby boosting the capture threshold.

The higher stimulation level also shortens battery life. This is notdesirable, as a shorter battery life necessitates more frequent surgeryto implant fresh batteries.

Another drawback is the potential for patient discomfort associated withthis higher stimulation level. This is because the higher stimulationlevel can stimulate the phrenic or diaphragmatic plexus or causeintercostal muscle pacing.

Lastly, the higher stimulation is less effective, due to entry block.

A need therefore exists for an ICD that can achieve reliable cardiaccapture with a lower stimulation level, thereby causing less damage tothe heart, extending battery life, causing less pain to the patient andhaving greater therapeutic effectiveness than current ICDs. A need alsoexists for an ICD that can better entrain the heart and can entrainportions of the heart from a greater distance.

SUMMARY OF THE INVENTION

It therefore is an object of the present invention to provide an ICDwith antitachycardial pacing capabilities, wherein the stimulation isadministered with a voltage either at, just above, or just below thediastolic depolarization threshold potential.

It is another object of the present invention to sense whether cardiaccapture has occurred, and if not, to administer additional stimulation.

It is another object of the present invention to provide the additionalstimulation at a slightly higher voltage level than that level ofstimulation which resulted in no capture.

It is another object of the present invention to repeat thestimulation—sensing cycle until cardiac capture has occurred.

It is another object of the present invention to provide stimulationusing a biphasic waveform.

The present invention accomplishes the above objectives by providing animplantable cardioverter-defibrillator with a unique constellation offeatures and capabilities. Protocols disclosed include:

1/biphasic stimulation administered at, or just above, the diastolicdepolarization threshold potential;

2/biphasic or conventional stimulation initiated at, or just above, thediastolic depolarization threshold potential, reduced, upon capture, tobelow threshold; and

3/biphasic or conventional stimulation administered at a level set justbelow the diastolic depolarization threshold potential.

As mentioned, the antitachycardial pacing protocols of the presentinvention can be used in conjunction with biphasic pacing. The methodand apparatus relating to biphasic pacing comprises a first and secondstimulation phase, with each stimulation phase having a polarity,amplitude, shape, and duration. In a preferred embodiment, the first andsecond phases have differing polarities. In one alternative embodiment,the two phases are of differing amplitude. In a second alternativeembodiment, the two phases are of differing duration. In a thirdalternative embodiment, the first phase is in a chopped wave form. In afourth alternative embodiment, the amplitude of the first phase isramped. In a fifth alternative embodiment the first phase isadministered over 200 milliseconds after completion of a cardiacbeating/pumping cycle. In a preferred alternative embodiment, the firstphase of stimulation is an anodal pulse at maximum subthresholdamplitude for a long duration, and the second phase of stimulation is acathodal pulse of short duration and high amplitude. It is noted thatthe aforementioned alternative embodiments can be combined in differingfashions. It is also noted that these alternative embodiments areintended to be presented by way of example only, and are not limiting.

Enhanced myocardial function is obtained through the biphasic pacing ofthe present invention. The combination of cathodal with anodal pulses ofeither a stimulating or conditioning nature, preserves the improvedconduction and contractility of anodal pacing while eliminating thedrawback of increased stimulation threshold. The result is adepolarization wave of increased propagation speed. This increasedpropagation speed results in superior cardiac contraction leading to animprovement in blood flow and in increased access to reentrant circuits.Improved stimulation at a lower voltage level also results in reductionin scar tissue buildup thereby reducing the tendency of the capturethreshold to rise; reduction in power consumption leading to increasedlife for pacemaker batteries; and decreased pain to the patient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate examples of methodologies for treatingarrhythmias.

FIG. 2 illustrates an implantable cardioverter/defibrillator accordingto an embodiment of the present invention.

FIG. 3 is a schematic representation of leading anodal biphasicstimulation.

FIG. 4 is a schematic representation of leading cathodal biphasicstimulation.

FIG. 5 is a schematic representation of leading anodal stimulation oflow level and long duration, followed by conventional cathodalstimulation.

FIG. 6 is a schematic representation of leading anodal stimulation oframped low level and long duration, followed by conventional cathodalstimulation.

FIG. 7 is a schematic representation of leading anodal stimulation oflow level and short duration, administered in series followed byconventional cathodal stimulation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to the use of antitachycardial pacing tobreak up arrhythmia in the atrium. FIGS. 1A through 1C illustrateexamples of methodologies for treating arrhythmias.

FIG. 1A illustrates a first methodology. Here, a sensor senses the onsetof arrhythmia 102. In a preferred embodiment, this sensor comprises anantitachycardial pacing algorithm. Biphasic stimulation is thenadministered 104. In varying embodiments, this stimulation is either at,or just above the diastolic depolarization threshold. The ICD determineswhether capture has occurred 106. If capture has not occurred, thenstimulation continues at a slightly higher level 108. Thisstimulation—capture check—boost stimulation cycle continues untilcapture occurs. If capture has occurred, then stimulation is continuedfor a predetermined period of time 110. In a preferred embodiment,stimulation is administered as long as the arrhythmia persists.

In a preferred embodiment, stimulation pulses are administered at 80 to100 percent of the intrinsic rate with an approximately one to twosecond pause between each set of stimulation pulses. Then either thenumber of pulses increases, or the timing between pulses is adjusted.For example, in a preferred embodiment, the first pulse sequence can beat 80 percent of the intrinsic heart rate, the second pulse sequence at82 percent, the third pulse sequence at 84 percent, and so on. In apreferred embodiment a plurality of feedback loops provide data suchthat the voltage can be adjusted to constantly skirt the capturethreshold. Stimulation is continued until the rhythm reverts.

FIG. 1B illustrates a second methodology. Here, a sensor senses theonset of arrhythmia 112. In varying embodiments of the second method,either biphasic or conventional stimulation is then administered 114.This stimulation level is set at or just above the diastolicdepolarization threshold potential. The ICD determines whether capturehas occurred 116. If capture has not occurred, then stimulationcontinues at a slightly higher level 118. This stimulation—capturecheck—boost stimulation cycle continues until capture occurs. If capturehas occurred, then stimulation is gradually and continuously reduced tobelow threshold, and continued 120. Then, if capture is lost, thestimulation is raised to a slightly higher level and is again graduallyand continuously reduced. This entire sequence is repeated, such thatthe stimulation level hovers as close as possible to the loweststimulation level which provides capture. Stimulation continues untilthe rhythm reverts, for example, when the antitachycardial pacingalgorithm determines that pacing is no longer necessary.

FIG. 1C illustrates a third methodology. Here, a sensor senses the onsetof arrhythmia 122. In varying embodiments of the third method, eitherbiphasic or conventional stimulation is then administered 124. Thisstimulation level is set just below the diastolic depolarizationthreshold potential. The ICD determines whether capture has occurred126. If capture has not occurred, then stimulation continues at aslightly higher level 128. This stimulation—capture check—booststimulation cycle continues until capture occurs. If capture hasoccurred, then stimulation is continued at below threshold level 130. Ifcapture is lost then the stimulation is raised to a slightly higherlevel and is gradually and continuously reduced. This entire sequence isrepeated, such that the stimulation level hovers as close as possible tothe lowest stimulation level which provides capture. Stimulationcontinues until the rhythm reverts, for example, when theantitachycardial pacing algorithm determines that pacing is no longernecessary.

Sensing

Sensing can be direct or indirect. For example, direct sensing can bebased on data from sensing electrodes. The ICD of the present inventionincludes sensing circuits/electronics to sense an arrhythmia through oneor more sensing and/or stimulating electrodes. The sensing electronicssense the cardiac activity as depicted by electrical signals. Forexample, as is known in the art, R-waves occur upon the depolarizationof ventricular tissue and P-waves occur upon the depolarization ofatrial tissue. By monitoring these electrical signals the control/timingcircuit of the ICD can determine the rate and regularity of thepatient's heart beat, and thereby determine whether the heart isundergoing arrhythmia. This determination can be made by determining therate of the sensed R-waves and/or P-waves and comparing this determinedrate against various reference rates.

Direct sensing can be based upon varying criteria; such as, but notlimited to, primary rate, sudden onset, and stability. The sole criteriaof a primary rate sensor is the heart rate. When applying the primaryrate criteria, if the heart rate should exceed a predefined level, thentreatment is begun. Sensing electronics set to sudden onset criteriaignore those changes which occur slowly, and initiate treatment whenthere is a sudden change such as immediate paroxysmal arrhythmia. Thistype of criteria would thus discriminate against sinus tachycardia.Stability of rate can also be an important criteria. For example,treatment with a ventricular device would not be warranted for a fastrate that varies, here treatment with an atrial device would beindicated.

In alternative embodiments, sensing can be indirect. Indirect sensingcan be based on any of various functional parameters such as arterialblood pressure, rate of the electrocardiogram deflections or theprobability density function (pdf) of the electrocardiogram. Forexample, whether or not to administer treatment can also be affected bypdf monitoring of the time the signal spends around the baseline.

Sensing can also be augmented by stimulating the atria and observing andmeasuring the consequent effects on atrial and ventricular function.

Thus, in a preferred embodiment, sensing electronics are based uponmultiple criteria. In addition, the present invention envisions devicesworking in more than one chamber such that appropriate treatment can beadministered to either the atrium or the ventricle in response tosensing electronics based upon a variety of criteria, including thosedescribed above as well as other criteria known to those skilled in theart.

Stimulation

Electrical stimulation is delivered via lead(s) or electrode(s). Theseleads can be epicardial (external surface of the heart) or endocardial(internal surface of the heart) or any combination of epicardial andendocardial. Leads are well known to those skilled in the art; see, forexample, U.S. Pat. No. 4,662,377 to Heilman et al., U.S. Pat. No.4,481,953 to Gold et al., and U.S. Pat. No. 4,010,758 to Rockland etal., each of which is herein incorporated by reference in its entirety.

Lead systems can be unipolar or bipolar. A unipolar lead has oneelectrode on the lead itself, the cathode. Current flows from thecathode, stimulates the heart, and returns to the anode on the casing ofthe pulse generator to complete the circuit. A bipolar lead has twopoles on the lead a short distance from each other at the distal end,and both electrodes lie within the heart. With the reference to FIG. 2,a system according to an embodiment of the present invention isillustrated. An automatic implantable cardioverter/defibrillator 2 isimplanted within the body of the patient and has a pair of outputterminals, an anode 4 and a cathode 6. The ICD 2 is coupled to aflexible catheter electrode arrangement 8 having a distal electrode 10and a proximal electrode 12, each associated with the patient's heart.Other electrode configurations may be employed, such as ring-typeelectrodes. As for external electrodes, an anodal electrode 24 may beemployed. The automatic ICD 2 includes sensing and detecting circuitry,as well as pulse generating circuitry, the output of the latter beingcoupled to the implantable electrodes 10, 12. The ICD 2 senses anarrhythmic condition of the heart and, in response thereto, issues oremits cardioverting or defibrillating pulses to the heart, through theimplantable electrodes 10, 12.

The catheter electrode 8 is inserted intravenously to a position suchthat the distal electrode 10 is positioned in the right ventricular apex14 of the heart and the proximal electrode 12 is positioned in thesuperior vena cava region 16 of the heart. It should be appreciatedthat, as the term is used herein, the superior vena cava 16 may alsoinclude portions of the right atrium 18.

Conventional stimulation is well known to those skilled in the art andcomprises monophasic waveforms (cathodal or anodal) as well asmultiphasic waveforms wherein the nonstimulating pulses are of a minimalmagnitude and are used, for example, to dissipate a residual charge onan electrode.

FIGS. 3 through 7 depict a range of biphasic stimulation protocols.These protocols have been disclosed in U.S. patent application No.08/699,552 to Mower, which is herein incorporated by reference in itsentirety.

FIG. 3 depicts biphasic electrical stimulation wherein a firststimulation phase comprising anodal stimulus 302 is administered havingamplitude 304 and duration 306. This first stimulation phase isimmediately followed by a second stimulation phase comprising cathodalstimulation 308 of equal intensity and duration.

FIG. 4 depicts biphasic electrical stimulation wherein a firststimulation phase comprising cathodal stimulation 402 having amplitude404 and duration 406 is administered. This first stimulation phase isimmediately followed by a second stimulation phase comprising anodalstimulation 408 of equal intensity and duration.

FIG. 5 depicts a preferred embodiment of biphasic stimulation wherein afirst stimulation phase, comprising low level, long duration anodalstimulation 502 having amplitude 504 and duration 506, is administered.This first stimulation phase is immediately followed by a secondstimulation phase comprising cathodal stimulation 508 of conventionalintensity and duration. In differing alternative embodiments, anodalstimulation 502 is: 1) at maximum subthreshold amplitude; 2) less thanthree volts; 3) of a duration of approximately two to eightmilliseconds; and/or 4) administered over 200 milliseconds post heartbeat. Maximum subthreshold amplitude is understood to mean the maximumstimulation amplitude that can be administered without eliciting acontraction. In a preferred embodiment, anodal stimulation isapproximately two volts for approximately three milliseconds duration.In differing alternative embodiments, cathodal stimulation 508 is: 1) ofa short duration; 2) approximately 0.3 to 1.5 milliseconds; 3) of a highamplitude; 4) in the approximate range of three to twenty volts; and/or5) of a duration less than 0.3 millisecond and at a voltage greater thantwenty volts. In a preferred embodiment, cathodal stimulation isapproximately six volts administered for approximately 0.4 millisecond.In the manner disclosed by these embodiments, as well as thosealterations and modifications which can become obvious upon the readingof this specification, a maximum membrane potential without activationis achieved in the first phase of stimulation.

FIG. 6 depicts an alternative preferred embodiment of biphasicstimulation wherein a first stimulation phase, comprising anodalstimulation 602, is administered over period 604 with rising intensitylevel 606. The ramp of rising intensity level 606 can be linear ornon-linear, and the slope can vary. This anodal stimulation isimmediately followed by a second stimulation phase comprising cathodalstimulation 608 of conventional intensity and duration. In alternativeembodiments, anodal stimulation 602: (1) rises to a maximum subthresholdamplitude less than three volts; (2) is of a duration of approximatelytwo to eight milliseconds; and/or (3) is administered over 200milliseconds post heart beat. In yet other alternative embodiments,cathodal stimulation 608 is: (1) of a short duration; (2) approximately0.3 to 1.5 milliseconds; (3) of a high amplitude; (4) in the approximaterange of three to twenty volts; and/or (5) of a duration less than 0.3milliseconds and at a voltage greater than twenty volts. In the mannerdisclosed by these embodiments, as well as those alterations andmodifications which can become obvious upon the reading of thisspecification, a maximum membrane potential without activation isachieved in the first phase of stimulation.

FIG. 7 depicts biphasic electrical stimulation wherein a firststimulation phase, comprising series 702 of anodal pulses, isadministered at amplitude 704. In one embodiment, rest period 706 is ofequal duration to stimulation period 708, and is administered atbaseline amplitude. In an alternative embodiment, rest period 706 is ofa differing duration than stimulation period 708, and is administered atbaseline amplitude. Rest period 706 occurs after each stimulation period708, with the exception that a second stimulation phase, comprisingcathodal stimulation 710 of conventional intensity and duration,immediately follows the completion of series 702. In alternativeembodiments: (1) the total charge transferred through series 702 ofanodal stimulation is at the maximum subthreshold level; and/or (2) thefirst stimulation pulse of series 702 is administered over 200milliseconds post heart beat. In yet other alternative embodiments,cathodal stimulation 710 is: (1) of a short duration; (2) approximately0.3 to 1.5 milliseconds; (3) of a high amplitude; (4) in the approximaterange of three to twenty volts, and/or (5) of a duration less than 0.3milliseconds and at a voltage greater than twenty volts.

Determining Cardiac Capture

Capture can be determined by multiple means. First, capture or the lossthereof, can be determined by monitoring cardiac rhythm. Loss of capturecan result in a change in timing of the heart beat.

Second, capture can be monitored through the development of a template.The template can be based on parameters such as electrocardiogram data,mechanical motion and/or probability density function data. Where thetemplate is established pre-stimulation, a change in the baselinesignifies capture. Where the template is established after capture hasoccurred, a change in the template characteristics signifies loss ofcapture. The templates can be established and/or updated at any time.

Once capture occurs the stimulation protocol of the entrained sites isadjusted as illustrated by FIGS. 1A through 1C.

Having thus described the basic concept of the invention, it will bereadily apparent to those skilled in the art that the foregoing detaileddisclosure is intended to be presented by way of example only, and isnot limiting. Various alterations, improvements and modifications willoccur and are intended to those skilled in the art, but are notexpressly stated herein. These modifications, alterations andimprovements are intended to be suggested hereby, and within the scopeof the invention. Further, the pacing pulses described in thisspecification are well within the capabilities of existing pacemakerelectronics with appropriate programming. Accordingly, the invention islimited only by the following claims and equivalents thereto.

What is claimed is:
 1. A method of operating an implantable cardiacstimulator to perform cardioverting, the cardiac stimulator havingoutput means for delivering electrical stimulation of a predeterminedpolarity, amplitude, shape and duration, the method comprising: sensingthe onset of tachycardia; applying pulses of biphasic pacing stimulationat a first intensity level selected from the group consisting of at thediastolic depolarization threshold, below the diastolic depolarizationthreshold or above the diastolic depolarization threshold, wherein eachpulse of biphasic pacing stimulation comprises: a first stimulationphase with a first phase polarity, a first phase amplitude, a firstphase shape and a first phase duration; and a second stimulation phasewith a second phase polarity, a second phase amplitude, a second phaseshape and a second phase duration; and determining whether pacingcapture has occurred; wherein in the event it is determined that capturehas occurred, the method further comprising: continuing biphasicstimulation for a predetermined period of time.
 2. The method ofoperating an implantable cardiac stimulator as in claim 1, wherein it isdetermined that capture has not occurred, further comprising: increasingthe stimulation intensity level by predefined increments until captureoccurs.
 3. The method of operating an implantable cardiac stimulator asin claim 1, wherein the first phase polarity is positive.
 4. The methodof operating an implantable cardiac stimulator as in claim 1, whereinthe first phase duration is at least as long as the second phaseduration.
 5. The method of operating an implantable cardiac stimulatoras in claim 1, wherein the first stimulation phase comprises an anodalstimulus.
 6. A method of operating an implantable cardiac stimulator toperform cardioverting, the cardiac stimulator having output means fordelivering electrical stimulation of a predetermined polarity,amplitude, shape and duration, the method comprising: sensing the onsetof tachycardia; applying pulses of biphasic pacing stimulation at afirst intensity level selected from the group consisting of at thediastolic depolarization threshold, below the diastolic depolarizationthreshold or above the diastolic depolarization threshold, wherein eachpulse of biphasic pacing stimulation comprises: a first stimulationphase with a first phase polarity, a first phase amplitude, a firstphase shape and a first phase duration; and a second stimulation phasewith a second phase polarity, a second phase amplitude, a second phaseshape and a second phase duration; and determining whether pacingcapture has occurred; wherein the first phase amplitude is less than thesecond phase amplitude.
 7. A method of operating an implantable cardiacstimulator to perform cardioverting, the cardiac stimulator havingoutput means for delivering electrical stimulation of a predeterminedpolarity, amplitude, shape and duration, the method comprising: sensingthe onset of tachycardia; applying pulses of biphasic pacing stimulationat a first intensity level selected from the group consisting of at thediastolic depolarization threshold, below the diastolic depolarizationthreshold or above the diastolic depolarization threshold, wherein eachpulse of biphasic pacing stimulation comprises: a first stimulationphase with a first phase polarity, a first phase amplitude, a firstphase shape and a first phase duration; and a second stimulation phasewith a second phase polarity, a second phase amplitude, a second phaseshape and a second phase duration; and determining whether pacingcapture has occurred; wherein the first phase amplitude is ramped from abaseline value to a second value.
 8. The method of operating animplantable cardiac stimulator as in claim 7, wherein the second valueis equal to the second phase amplitude.
 9. The method of operating animplantable cardiac stimulator as in claim 7, wherein the second valueis at a maximum subthreshold amplitude.
 10. The method of operating animplantable cardiac stimulator as in claim 9, wherein the maximumsubthreshold amplitude is about 0.5 to 3.5 volts.
 11. The method ofoperating an implantable cardiac stimulator as in claim 7, wherein thefirst phase duration is at least as long as the second phase duration.12. The method of operating an implantable cardiac stimulator as inclaim 7, wherein the first phase duration is about one to ninemilliseconds.
 13. The method of operating an implantable cardiacstimulator as in claim 7, wherein the second phase amplitude is abouttwo volts to twenty volts.
 14. The method of operating an implantablecardiac stimulator as in claim 7, wherein the second phase duration isabout 0.2. to 0.9 milliseconds.
 15. The method of operating animplantable cardiac stimulator as in claim 7, wherein the second phaseduration is less than 0.3 milliseconds and the second phase amplitude isgreater than 20 volts.
 16. A method of operating an implantable cardiacstimulator to perform cardioverting, the cardiac stimulator havingoutput means for delivering electrical stimulation of a predeterminedpolarity, amplitude, shape and duration, the method comprising: sensingthe onset of tachycardia; applying pulses of biphasic pacing stimulationat a first intensity level selected from the group consisting of at thediastolic depolarization threshold, below the diastolic depolarizationthreshold or above the diastolic depolarization threshold, wherein eachpulse of biphasic pacing stimulation comprises: a first stimulationphase with a first phase polarity, a first phase amplitude, a firstphase shape and a first phase duration; and a second stimulation phasewith a second phase polarity, a second phase amplitude, a second phaseshape and a second phase duration; and determining whether pacingcapture has occurred; wherein the first stimulation phase furthercomprises a series of stimulating pulses of a predetermined amplitude,polarity and duration.
 17. The operating an implantable cardiacstimulator as in claim 16, wherein the first stimulation phase furthercomprises a series of rest periods.
 18. The method of operating animplantable cardiac stimulator as in claim 17, wherein applying thefirst stimulation phase further comprises applying a rest period of abaseline amplitude after at least one stimulating pulse.
 19. The methodof operating an implantable cardiac stimulator as in claim 18, whereinthe rest period is of equal duration to the duration of the stimulatingpulse.
 20. A method of operating an implantablecardioverter-defibrillator (ICD), the ICD having output means fordelivering electrical stimulation of a predetermined polarity,amplitude, shape and duration, the method comprising: sensing the onsetof arrhythmia; applying biphasic stimulation at a first intensity levelselected from the group consisting of at the diastolic depolarizationthreshold, below the diastolic depolarization threshold or above thediastolic depolarization threshold wherein biphasic stimulationcomprises: a first stimulation phase with a first phase polarity, afirst phase amplitude, a first phase shape and a first phase duration,and a second stimulation phase with a second phase polarity, a secondphase amplitude, a second phase shape and a second phase duration; anddetermining whether capture has occurred; wherein the first phaseamplitude is ramped from a baseline value to a second value; wherein thefirst stimulation phase further comprises: a series of stimulatingpulses of a predetermined amplitude, polarity and duration; and a seriesof rest periods; wherein applying the first stimulation phase furthercomprises applying a rest period of a baseline amplitude after at leastone stimulating pulse; and wherein the rest period is of equal durationto the duration of the stimulating pulse.
 21. The method of operating anICD as in claim 20, wherein the first stimulation phase comprises ananodal stimulus.
 22. A method of operating an implantable cardiacstimulator to perform cardioverting, the cardiac stimulator havingoutput means for delivering electrical stimulation of a predeterminedpolarity, amplitude, shape and duration, the method comprising: sensingthe onset of tachycardia; applying pulses of biphasic pacing stimulationat a first intensity level selected from the group consisting of at thediastolic depolarization threshold, below the diastolic depolarizationthreshold or above the diastolic depolarization threshold, wherein eachpulse of biphasic pacing stimulation comprises: a first stimulationphase with a first phase polarity, a first phase amplitude, a firstphase shape and a first phase duration; and a second stimulation phasewith a second phase polarity, a second phase amplitude, a second phaseshape and a second phase duration; and determining whether pacingcapture has occurred; wherein the first phase amplitude is at a maximumsubthreshold amplitude.
 23. The method of operating an implantablecardiac stimulator as in claim 22, wherein the maximum subthresholdamplitude is about 0.5 to 3.5 volts.
 24. A method of operating animplantable cardiac stimulator to perform cardioverting, the cardiacstimulator having output means for delivering electrical stimulation ofa predetermined polarity, amplitude, shape and duration, the methodcomprising: sensing the onset of tachycardia; applying pulses ofbiphasic pacing stimulation at a first intensity level selected from thegroup consisting of at the diastolic depolarization threshold, below thediastolic depolarization threshold or above the diastolic depolarizationthreshold, wherein each pulse of biphasic pacing stimulation comprises:a first stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; anddetermining whether pacing capture has occurred; wherein the first phaseduration is about one to nine milliseconds.
 25. A method of operating animplantable cardiac stimulator to perform cardioverting, the cardiacstimulator having output means for delivering electrical stimulation ofa predetermined polarity, amplitude, shape and duration, the methodcomprising: sensing the onset of tachycardia; applying pulses ofbiphasic pacing stimulation at a first intensity level selected from thegroup consisting of at the diastolic depolarization threshold, below thediastolic depolarization threshold or above the diastolic depolarizationthreshold, wherein each pulse of biphasic pacing stimulation comprises:a first stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; anddetermining whether pacing capture has occurred; wherein the secondphase duration is about 0.2 to 0.9 milliseconds.
 26. A method ofoperating an implantable cardiac stimulator to perform cardioverting,the cardiac stimulator having output means for delivering electricalstimulation of a predetermined polarity, amplitude, shape and duration,the method comprising: sensing the onset of tachycardia; applying pulsesof biphasic pacing stimulation at a first intensity level selected fromthe group consisting of at the diastolic depolarization threshold, belowthe diastolic depolarization threshold or above the diastolicdepolarization threshold, wherein each pulse of biphasic pacingstimulation comprises: a first stimulation phase with a first phasepolarity, a first phase amplitude, a first phase shape and a first phaseduration; and a second stimulation phase with a second phase polarity, asecond phase amplitude, a second phase shape and a second phaseduration; and determining whether pacing capture has occurred; whereinthe second phase amplitude is about two volts to twenty volts.
 27. Amethod of operating an implantable cardiac stimulator to performcardioverting, the cardiac stimulator having output means for deliveringelectrical stimulation of a predetermined polarity, amplitude, shape andduration, the method comprising: sensing the onset of tachycardia;applying pulses of biphasic pacing stimulation at a first intensitylevel selected from the group consisting of at the diastolicdepolarization threshold, below the diastolic depolarization thresholdor above the diastolic depolarization threshold, wherein each pulse ofbiphasic pacing stimulation comprises: a first stimulation phase with afirst phase polarity, a first phase amplitude, a first phase shape and afirst phase duration; and a second stimulation phase with a second phasepolarity, a second phase amplitude, a second phase shape and a secondphase duration; and determining whether pacing capture has occurred;wherein the second phase duration is less than 0.3 milliseconds and thesecond phase amplitude is greater than 20 volts.
 28. A method ofoperating an implatable cardioverter-defibrillator (ICD), the ICD havingoutput means for delivering electrical stimulation of a predeterminedpolarity, amplitude, shape and duration, the method comprising: sensingthe onset of arrhythmia; applying biphasic stimulation at a firstintensity level selected from the group consisting of at the diastolicdepolarization threshold, below the diastolic depolarization thresholdor above the diastolic depolarization threshold wherein biphasicstimulation comprises: a first stimulation phase with a first phasepolarity, a first phase amplitude, a first phase shape and a first phaseduration, and a second stimulation phase with a second phase polarity, asecond phase amplitude, a second phase shape and a second phaseduration; and determining whether capture has occurred; wherein thefirst phase amplitude is ramped from a baseline value to a second value;and wherein the second phase duration is about 0.2 to 0.9 milliseconds.29. The method of operating an ICD as in claim 28, wherein the firststimulation phase comprises an anodal stimulus.
 30. A method ofoperating an implantable cardioverter-defibrillator (ICD), the ICDhaving output means for delivering electrical stimulation of apredetermined polarity, amplitude, shape and duration, the methodcomprising: sensing the onset of arrhythmia; applying biphasicstimulation at a first intensity level selected from the groupconsisting of at the diastolic depolarization threshold, below thediastolic depolarization threshold or above the diastolic depolarizationthreshold wherein biphasic stimulation comprises: a first stimulationphase with a first phase polarity, a first phase amplitude, a firstphase shape and a first phase duration, and a second stimulation phasewith a second phase polarity, a second phase amplitude, a second phaseshape and a second phase duration; and determining whether capture hasoccurred; wherein the first phase amplitude is ramped from a baselinevalue to a second value; and wherein the second phase duration is lessthan 0.3 milliseconds and the second phase amplitude is greater than 20volts.
 31. The method of operating an ICD as in claim 30, wherein thefirst stimulation phase comprises an anodal stimulus.
 32. A method ofoperating an implantable cardioverter-defibrillator (ICD), the ICDhaving output means for delivering electrical stimulation of apredetermined polarity, amplitude, shape and duration, the methodcomprising: sensing the onset of arrhythmia; applying biphasicstimulation at a first intensity level selected from the groupconsisting of at the diastolic depolarization threshold, below thediastolic depolarization threshold or above the diastolic depolarizationthreshold wherein biphasic stimulation comprises: a first stimulationphase with a first phase polarity, a first phase amplitude, a firstphase shape and a first phase duration, and a second stimulation phasewith a second phase polarity, a second phase amplitude, a second phaseshape and a second phase duration; and determining whether capture hasoccurred; wherein the first stimulation phase is initiated greater than200 milliseconds after completion of a cardiac beating cycle.
 33. Themethod of operating an ICD as in claim 32, wherein the first stimulationphase comprises an anodal stimulus.
 34. A method of operating animplantable cardioverter-defibrillator )ICD), the ICD having outputmeans for delivering electrical stimulation of a predetermined polarity,amplitude, shape and duration, the method comprising: sensing the onsetof arrhythmia; applying stimulation selected from the group consistingof biphasic stimulation and conventional stimulation at a firstintensity level selected from the group consisting of at the diastolicdepolarization threshold, below the diastolic depolarization thresholdor above the diastolic depolarization threshold; determining whethercapture has occurred; increasing the stimulation intensity level bypredefined increments until capture does occurs; and upon capture,continuing stimulation selected from the group consisting of biphasicstimulation and conventional stimulation at a second intensity levelbelow the diastolic depolarization threshold.
 35. A method of operatingan implantable cardioverter-defibrillator (ICD), the ICD having outputmeans for delivering electrical stimulation of a predetermined polarity,amplitude, shape and duration, the method comprising: defining a firststimulation phase with a positive polarity, a first phase amplitude, afirst phase shape and a first phase duration, wherein said first phaseamplitude is about 0.5 to 3.5 volts, wherein said first phase durationis about one to nine milliseconds and wherein said first stimulationphase is initiated greater than 200 milliseconds after completion of acardiac beating cycle; defining a second phase with a negative polarity,a second phase amplitude, a second phase shape and a second phaseduration, wherein said second phase amplitude is about four volts totwenty volts and wherein said second phase duration is about 0.2 to 0.9milliseconds; and sensing the onset of arrhythmia; applying the firststimulation phase and the second stimulation phase in sequence to thecardiac tissue; determining whether capture has occurred; and increasingthe stimulation intensity level by predefined increments until captureoccurs.
 36. A cardiac stimulator to perform cardioverting, the cardiacstimulator comprising: sensing means for sensing the onset oftachycardia; output means for delivering, in response to the sensingmeans, electrical stimulation of a predetermined polarity, amplitude,shape and duration to cause application of pulses of biphasic pacingstimulation at a first intensity level selected from the groupconsisting of: at the diastolic depolarization threshold, below thediastolic depolarization threshold, and above the diastolicdepolarization threshold; and means for determining whether capture hasoccurred; wherein each pulse of biphasic pacing stimulation comprises: afirst stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; and whereinin the event that the means for determining determines that capture hasoccurred, the output means continues biphasic stimulation for apredetermined period of time.
 37. The cardiac stimulator as in claim 36,wherein in the event that the means for determining determines thatcapture has not occurred, the output means increases the stimulationintensity level by predefined increments until capture occurs.
 38. Thecardiac stimulator as in claim 36, wherein the first stimulation phasecomprises an anodal stimulus.
 39. The cardiac stimulator as in claim 36,wherein the first phase duration is at least as long as the second phaseduration.
 40. A cardiac stimulator to perform cardioverting, the cardiacstimulator comprising: sensing means for sensing the onset oftachycardia; output means for delivering, in response to the sensingmeans, electrical stimulation of a predetermined polarity, amplitude,shape and duration to cause application of pulses of biphasic pacingstimulation at a first intensity level selected from the groupconsisting of: at the diastolic depolarization threshold, below thediastolic depolarization threshold, and above the diastolicdepolarization threshold; and means for determining whether capture hasoccurred; wherein each pulse of biphasic pacing stimulation comprises: afirst stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; and whereinthe first phase amplitude is less than the second phase amplitude.
 41. Acardiac stimulator to perform cardioverting, the cardiac stimulatorcomprising: sensing means for sensing the onset of tachycardia; outputmeans for delivering, in response to the sensing means, electricalstimulation of a predetermined polarity, amplitude, shape and durationto cause application of pulses of biphasic pacing stimulation at a firstintensity level selected from the group consisting of: at the diastolicdepolarization threshold, below the diastolic depolarization threshold,and above the diastolic depolarization threshold; and means fordetermining whether capture has occurred; wherein each pulse of biphasicpacing stimulation comprises: a first stimulation phase with a firstphase polarity, a first phase amplitude, a first phase shape and a firstphase duration; and a second stimulation phase with a second phasepolarity, a second phase amplitude, a second phase shape and a secondphase duration; and wherein the first phase amplitude is ramped from abaseline value to a second value.
 42. The cardiac stimulator as in claim41, wherein the second value is equal to the second phase amplitude. 43.The cardiac stimulator as in claim 41, wherein the second value is at amaximum subthreshold amplitude.
 44. The cardiac stimulator as in claim43, wherein the maximum subthreshold amplitude is about 0.5 to 3.5volts.
 45. The cardiac stimulator as in claim 41, wherein the firstphase duration is at least as long as the second phase duration.
 46. Thecardiac stimulator as in claim 44, wherein the first phase duration isabout one to nine milliseconds.
 47. The cardiac stimulator as in claim44, wherein the second phase duration is about 0.2 to 0.9 milliseconds.48. The cardiac stimulator as in claim 41, wherein the second phaseamplitude is about two volts to twenty volts.
 49. The cardiac stimulatoras in claim 41, wherein the second phase duration is less than 0.3milliseconds and the second phase amplitude is greater than 20 volts.50. A cardiac stimulator to perform cardioverting, the cardiacstimulator comprising: sensing means for sensing the onset oftachycardia; output means for delivering, in response to the sensingmeans, electrical stimulation of a predetermined polarity, amplitude,shape and duration to cause application of pulses of biphasic pacingstimulation at a first intensity level selected from the groupconsisting of: at the diastolic depolarization threshold, below thediastolic depolarization threshold, and above the diastolicdepolarization threshold; and means for determining whether capture hasoccurred; wherein each pulse of biphasic pacing stimulation comprises: afirst stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; and whereinthe first stimulation phase further comprises a series of stimulatingpulses of a predetermined amplitude, polarity and duration.
 51. Thecardiac stimulator as in claim 50, wherein the first stimulation phasefurther comprises a series of rest periods.
 52. The cardiac stimulatoras in claim 51, wherein applying the first stimulation phase furthercomprises applying a rest period of a baseline amplitude after at leastone stimulating pulse.
 53. The cardiac stimulator as in claim 52,wherein the rest period is of equal duration to the duration of thestimulating pulse.
 54. A cardiac stimulator to perform cardioverting,the cardiac stimulator comprising: sensing means for sensing the onsetof tachycardia; output means for delivering, in response to the sensingmeans, electrical stimulation of a predetermined polarity, amplitude,shape and duration to cause application of pulses of biphasic pacingstimulation at a first intensity level selected from the groupconsisting of: at the diastolic depolarization threshold, below thediastolic depolarization threshold, and above the diastolicdepolarization threshold; and means for determining whether capture hasoccurred; wherein each pulse of biphasic pacing stimulation comprises: afirst stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; and whereinthe first phase amplitude is at a maximum subthreshold amplitude. 55.The cardiac stimulator as in claim 54, wherein the maximum subthresholdamplitude is about 0.5 to 3.5 volts.
 56. A cardiac stimulator to performcardioverting, the cardiac stimulator comprising: sensing means forsensing the onset of tachycardia; output means for delivering, inresponse to the sensing means, electrical stimulation of a predeterminedpolarity, amplitude, shape and duration to cause application of pulsesof biphasic pacing stimulation at a first intensity level selected fromthe group consisting of: at the diastolic depolarization threshold,below the diastolic depolarization threshold, and above the diastolicdepolarization threshold; and means for determining whether capture hasoccurred; wherein each pulse of biphasic pacing stimulation comprises: afirst stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; and whereinthe first phase duration is about one to nine milliseconds.
 57. Acardiac stimulator to perform cardioverting, the cardiac stimulatorcomprising: sensing means for sensing the onset of tachycardia; outputmeans for delivering, in response to the sensing means, electricalstimulation of a predetermined polarity, amplitude, shape and durationto cause application of pulses of biphasic pacing stimulation at a firstintensity level selected from the group consisting of: at the diastolicdepolarization threshold, below the diastolic depolarization threshold,and above the diastolic depolarization threshold; and means fordetermining whether capture has occurred; wherein each pulse of biphasicpacing stimulation comprises: a first stimulation phase with a firstphase polarity, a first phase amplitude, a first phase shape and a firstphase duration; and a second stimulation phase with a second phasepolarity, a second phase amplitude, a second phase shape and a secondphase duration; and wherein the second phase duration is about 0.2 to0.9 milliseconds.
 58. A cardiac stimulator to perform cardioverting, thecardiac stimulator comprising: sensing means for sensing the onset oftachycardia; output means for delivering, in response to the sensingmeans, electrical stimulation of a predetermined polarity, amplitude,shape and duration to cause application of pulses of biphasic pacingstimulation at a first intensity level selected from the groupconsisting of: at the diastolic depolarization threshold, below thediastolic depolarization threshold, and above the diastolicdepolarization threshold; and means for determining whether capture hasoccurred; wherein each pulse of biphasic pacing stimulation comprises: afirst stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; and whereinthe second phase amplitude is about two volts to twenty volts.
 59. Acardiac stimulator to perform cardioverting, the cardiac stimulatorcomprising: sensing means for sensing the onset of tachycardia; outputmeans for delivering, in response to the sensing means, electricalstimulation of a predetermined polarity, amplitude, shape and durationto cause application of pulses of biphasic pacing stimulation at a firstintensity level selected from the group consisting of: at the diastolicdepolarization threshold, below the diastolic depolarization threshold,and above the diastolic depolarization threshold; and means fordetermining whether capture has occurred; wherein each pulse of biphasicpacing stimulation comprises: a first stimulation phase with a firstphase polarity, a first phase amplitude, a first phase shape and a firstphase duration; and a second stimulation phase with a second phasepolarity, a second phase amplitude, a second phase shape and a secondphase duration; and wherein the second phase duration is less than 0.3milliseconds and the second phase amplitude is greater than 20 volts.60. A cardiac stimulator to perform cardioverting, the cardiacstimulator comprising: sensing means for sensing the onset oftachycardia; output means for delivering, in response to the sensingmeans, electrical stimulation of a predetermined polarity, amplitude,shape and duration to cause application of pulses of biphasic pacingstimulation at a first intensity level selected from the groupconsisting of: at the diastolic depolarization threshold, below thediastolic depolarization threshold, and above the diastolicdepolarization threshold; and means for determining whether capture hasoccurred; wherein each pulse of biphasic pacing stimulation comprises: afirst stimulation phase with a first phase polarity, a first phaseamplitude, a first phase shape and a first phase duration; and a secondstimulation phase with a second phase polarity, a second phaseamplitude, a second phase shape and a second phase duration; and whereinthe first stimulation phase is initiated greater than 200 millisecondsafter completion of a cardiac beating cycle.
 61. An implantable cardiacstimulator device comprising: plural electrodes; sensing circuitryconnected to the plural electrodes and adapted to sense the onset oftachycardia; detecting circuitry connected to the sensing circuitry andadapted to detect whether pacing capture has occurred; and pulsegenerating circuitry connected to the plural electrodes and adapted togenerate, in response to the sensing circuitry, electrical pulses of apredetermined polarity, amplitude, shape and duration to causeapplication of pulses of biphasic pacing stimulation at a firstintensity level selected from the group consisting of: at the diastolicdepolarization threshold, below the diastolic depolarization threshold,and above the diastolic depolarization threshold; and wherein each pulseof biphasic pacing stimulation comprises: a first stimulation phase witha first phase polarity, a first phase amplitude, a first phase shape anda first phase duration; and a second stimulation phase with a secondphase polarity, a second phase amplitude, a second phase shape and asecond phase duration; and wherein, in the event that the detectingcircuitry determines that capture has occurred, the pulse generatingcircuitry continues biphasic stimulation for a predetermined period oftime.
 62. The implantable cardiac stimulator device as in claim 61,wherein, in the event that the detecting circuitry determines thatcapture has not occurred, the pulse generating circuitry increases thestimulation intensity level by predefined increments until captureoccurs.